THE  MEASUREMENT  OF  TRANSLU- 
GENCY  OF  CERAMIC  BODIES  BY 
THE  USE  OF  A PHOTO-ELEC- 
TRIC CELL 

BY 

ROY  E.  LOWRANCE 


THESIS 

FOB  THE 

DEGREE  OF  BACHELOR  OF  SCIENCE 

IN 

CERAMIC  ENGINEERING 


COLLEGE  OF  ENGINEERING 

UNIVERSITY  OF  ILLINOIS 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/measurementoftraOOIora 


TABLE  OF  CONTENTS 
******* 

Page 

I . INTRODUCTION.  1 

II.  OTHER  METHODS  USED  IN  MEASURING  TRANSDUCER CY 2 

III  .EXPERIMENTAL  5 

1.  The  Photo-electric  Effect 5 

2 . Description  of  Apparatus 5 

(a)  The  Photo-electric  Cell............. 5 

fhj  The  Galvanometer  7 

(c)  Voltage............. 7 

(d)  Set-up  of  Apparatus 

3 . Preparation  of  Bodies  7 

4.  Preparation  of  Specimens... 8 

5.  Method  of  Procedure ...  9 

IV . RESULTS 11 

V.  CONCLUSIONS..... 14 

VI . BIBLIOGRAPHY 15 

VI I.  REFERENCES 16 


THE  MEASUREMENT  OF  TRANSLUCENCY  OF  CERAM I C BODIES 
BY  THE  USE  OF  A PHOTOELECTRIC  CELL 

******  x-  * * * * * **  * * * * 

I.  INTRODUCTION 

The  old  method  of  measuring  translucency  in  which  a potter 
puts  his  finger  "behind  a plate  and  observes  its  shadow,  is  acknowledge 
ed  by  all  to  be  very  crude  and  unsatisfactory . Various  other 
methods  have  been  proposed  but  there  seems  to  be  no  agreement  among 
the  various  experimenters,  as  to  which  method  is  the  best.  Most 
of  these  methods  depend  upon  the  sliding  of  a wedge-shaped  specimen 
past  a slot,  behind  which  is  a light  of  a certain  power  and  marking 
the  point  on  the  wedge  where  the  light  just  disappears.  The 
thickness  of  the  specimen  at  that  point  is  taken  as  a numerical 
expression  of  the  translucency . It  can  be  readily  seen  that  such 
methods  are  not  a real  measure  of  translucency , but  only  a relative 
measurement  since  there  is  no  end  point  and  the  ability  of  various 
people  to  judge  the  disappearance  of  a light  would  introduce  a 
personal  factor  which  could  not  be  corrected  for. 

Tne  object  of  this  worx  is  to  devise  a method  of  measuring 
translucency  by  the  use  of  a photoelectric  cell. 


II.  OTHER  METHODS  USED  I IT  MEASURING  TRANSDUCER  CY 

Various  methods  which  have  been  used  in  measuring 
translucency  of  ceramic  wares  are  as  follows: 

Ashley  1 held  wedge  shaped  pieces  close  to  an  electric 
or  incandescent  light  and  outlined  the  translucent  area  with  a 

pencil.  The  thickness  is  then  calipered. 

2 

Ogden  measured  the  translucency  of  the  thickness  of 
the  body  necessary  to  shut  out  the  light  of  a 16-candle-power  in- 
cadescent  lamp.  A box  of  sufficient  size  held  an  electric  lamp. 

One  side  of  the  box  was  of  zinc,  in  which  there  was  a narrow  slit 
about  half  an  inch  long.  Wedge  shaped  pieces  were  moved  up  and 
down  past  this  slit  until  the  light  was  just  shut  off.  The 

thickness  of  this  point  on  the  wedge  was  then  calipered. 

3 

V/illiams  and  Ashley  used  a different  method  than  any  of 
the  above  mentioned  in  measuring  the  translucency  of  white  ware 
samples.  A series  of  screens  which  varied  in  size  from  l/ 4 inch 
downward  were  placed  at  a distance  of  about  3 inches  from  an 
incadescent  light,  with  a reflector.  Samples  of  white  ware  were 
held  flat  against  the  wires  . The  smallest  mesh  distinguishable 
through  a specimen  was  noted  and  the  thickness  of  the  piece 
measured.  They  proposed  that  a standard  test  for  translucency 
be  established,  using  a standard  series  of  screens  with  regular 
variations  in  size  with  a constant  ratio  in  the  diameter  of  the 
wire  and  width  of  opening.  A lamp  of  stands.rd  intensity  would  also 


be  used. 


- 3 - 

4 

Parmelee  and  Baldwin  placed  a lamp  bulb  within  and 
immediately  below  a slot  cut  in  the  top  0f  a wooden  box.  Wedge 
shaped  specimens  were  placed  over  the  slot  and  it  was  possible  to 
determine  when,  in  the  scale  of  mixtures,  translucency  began  and 
whether  it  increased  or  decreased  with  varying  compositions.  A 
fine  wire  was  also  interposed  between  thelight  ana  the  trial  pieces. 
This  wire  could  be  clearly  seen  while  in  a stationary  position  with 
pieces  of  high  translucency,  but  in  pieces  of  low  translucency  it 
could  not  be  detected  unless  it  was  moved  across  the  illuminated 
field . 

5 

Watts  determined  the  translucency  of  wedge  shaped  speci- 
mens by  placing  them  over  a 1”  hole  in  a box  containing  a 16  candle 
power  incadescent  lamp  of  constant  brilliancy.  The  maximum 
thickness  of  the  trial  piece  expressed  in  centimeters,  through 
which  a No.  2C  wire  could  be  detected  on  the  face  of  the  trial 
next  the  lamp,  with  the  lamp  3 inches  distant,  was  taken  for  the 

measurement  of  translucency. 

6 

Lin  arranged  wedges  of  different  body  mixtures  in  a row. 

A good  light  was  placed  behind  them.  A metal  wire  about  1 mm. 
in  diameter  was  held  close  to  the  lighted  side  of  the  wedge  under 
examination.  The  thickness  of  the  wedge,  at  the  point  below  which 

the  wire  became  invisible,  was  measured  by  means  of  an  Arnes  dial. 

7 

Bleininger  suggested  that  translucency  might  be  measured 
oy  means  of  a selenium  cell  which  has  the  property  of  having  its 
conductivity  lowered  as  light  falls  upon  it.  Such  a method  is 
used  in  astronomy  for  measuring  and  comparing  the  various  light 


• 

. 


-4- 


intensities  of  stars.  No  details  of  the  plan  were  worked  out. 

8 

Priest  describes  an  apparatus  used  in  grading  tracing 
cloth,  by  covering  a standard  black  and  white  panel  with  a piece  of 
tracing  cloth,  and  with  a suitable  photometer,  measuring  the  ratio 
of  the  brightness  of  each  color.  It  was  advocated  that  this 
method  could  be  used  to  measure  the  translucency  of  porcelain  disks, 
but  the  details  were  not  worked  out. 

A method  of  measuring  translucency  by  means  of  a photometer, 
is  advocated  in  "Pottery  Industry",  Department  of  Commerce 
Miscellaneous  Series,  No.  21.  The  variation  in  translueency  is 
represented  by  the  light  transmitted  through  the  specimen  from 
a standard  lamp.  This  is  expressed  in  percentage  of  the  candle- 
power  of  the  standard.  A thickness  correction  affords  only  a 
rough  approximation,  since  it  is  evident  that  the  light  transmission 
is  not  diminished  in  direct  proportion  of  the  specimen.  The  true 
correction  factor  has  not  yet  been  established. 


-5- 

III.  EXPERIMENTAL 

1 • The  Photoelectric  Effect...  The  emission  of  ja-ega-t-ive 
electrons  from  an  illuminated,  plate  is  generally  kn own  as  the  photo- 
electric effect.  The  alkali  metals  are  particularly  sensitive 
and  for  this  reason  are  used  in  measuring  light  of  low  intensity. 

The  rate  at  which  these  negative  electrons  are 
emitted  is  variable  depending  upon  the  pressure,  nature  of  the  gas 
around  the  plate,  state  of  polish  of  the  surface  and  the  length 
of  exposure. 

The  current  produced  can  be  largely  increased  by 
making  use  of  ionization  by  collision  in  the  surrounding  gas.  Cells 
containing  some  inert  gas,  such  as  argon,  have  been  used  to  measure 
the  light  radiation  from  a candle  at  a distance  of  three  miles. 

2 . Description  of  Apparatus.- 

(a)  The  Photoelectric  Cell.  The  photoelectric 
cell  used  in  the  experiment  was  made  by  Dr. Jacob  K.unz  of  the  Depart- 
ment of  Physics,  University  of  Illinois.  Silver  was  vaporized 
and  then  allowed  to  condense  in  a thin  film  over  all  of  the  inside 
of  the  cell  except  a round  opening  for  the  admittance  of  light. 
Potassium  was  then  vaporized  and  upon  condensing  a thin  film  was 
formed  over  the  silver  film.  This  formed  the  cathode  or  negative 
terminal  of  the  cell. 

The  anode  consisted  of  a hoop  of  platinum  wire, 
directly  in  front  and  a few  centimeters  from  the  cathode. 

Tne  cell  was  filled  with  argon,  a gas  which  is 
generally  used  when  very  sensitive  measurements  are  to  be  made. 


-6- 


A resistance  of  100,000  ohms  was  connected  in  series  with 
the  cell,  in  order  to  protect  it  from  a sudden  increase  in  voltage. 

The  action  of  the  cell  was  as  follows,  light  was  allowed 
to  strike  upon  the  cathode  and  negative  electrons  were  given  off 
from  the  potassium.  When  a difference  of  potential  was  maintained 
between  the  anode  and  the  cathode,  these  negative  ions  flowed 
toward  the  anode.  The  current  produced  w as  measured  by  a sensitive 
galvanometer. 

The  photoelectric  cell  was  placed  in  a small  light-tight- 
box,  12  in.  by  6 in.  by  3 in.  A round  opening  7/8  in.  in  diameter 
was  made  in  one  side  of  the  box.  This  opening  could  be  closed 
and  opened  at  will  by  means  of  a sliding  shutter,  allowing  the 
light  to  fall  upon  the  cathode  of  the  cell. 

This  box  was  placed  in  a large  wooden  box  which  was  also 
light-tight  and  painted  black  inside.  This  larger  box  measured 
2 l/2  ft.  by  2 ft.  by  2 ft.  and  contained  in  addition  to  the  photo- 
electric cell,  the  source  of  illumination  and  a lens  to  make  the 
rays  of  light  parallel. 

The  source  of  light  was  a Mazda  25  watt  point  bulb.  Tor 
specimens  of  low  translucency  a stronger  light  would  have  been 
preferrable.  The  light  rays  were  made  parallel  by  means  of  a lens 
which  focused  the  light  strongly  upon  the  7/3  in.  opening  of  the 
box  containing  the  photoelectric  cell.  It  was  found  that  best 
results  were  obtained  with  the  light  18  inches  from  the  photoelectric 
cell  and  the  lens  bet ween  the  two , its  focal  length  being  such 
as  to  focus  the  light  sharply  upon  the  specimen. 


-7- 

(b)  The  Galvanomet er. - The  galvanometer  was  of  the  D* 
Arsenval  type  with  the  following  characteristics  : 

Sensitiveness  - 2.711  megs,  or  a figure  of  merit  of 

, , n - 12 
about  10 

Period  - 6.3  seconds 

Resistance  - 500  ohms. 

(c)  Voltage . - The  source  of  voltage  consisted  of 
flat  flash-light  batteries  of  the  French  four-cell  type,  connected 
in  series.  Each  cell  had  a strength  of  4.5  volts. 

(d)  Set-up  of  Apparatus.-  The  anode  of  the  photoelectric 
cell  was  connected  in  series  with  a resistance  coil,  having  a 
resistance  of  100,000  ohms,  then  directly  to  the  galvanometer. 

The  cathode  was  connected  directly  through  a mercury 
switch  to  the  negative  end  of  the  source  of  voltage.  This  mercury 
switch  made  it  possible  to  turn  the  current  on  and  off  easily  and 
quickly.  The  positive  end  of  the  flash-light  cells  was  connected 
directly  to  the  galvanometer. 

A damping  key  was  of  great  aid  in  shortening  the 
swing  of  the  galvanometer  and  enabled  a reading  to  be  made  quickly 
and  accurately. 

3 . Preparation  of  Bodies.-  The  composition  of  the 

body  mixtures  as  shown  in  Table  No.  I,  consisted  of  feldspar  from 

Pottery,  . 

Abingdon*  flint  and  clay.  The  clay  content  was  made  up  of  1/3 

Tennessee  ball  clay  No. 3.,  l/3  h-arris  North  Carolina  Kaolin  and 

l/3  Florida  kaolin. 

A 2-kilogram  batch  of  each  corner  body  v/as  we ighed 


1 

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40 

15 

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35 

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2/3 

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10 

20 

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21 

2/3 

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2/3 

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12 

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1/3 

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2/3 

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2/3 

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Corn position  of  Bo  d/ea 


$ 

SO 


C/ay 


-8- 


up , i.e.,  Nos.  1,  5,  6,  9 and  10.  Three  thousand  cc.  of  distilled 
water  were  added  and  the  whole  was  ground  in  a ball  mill  for  two 
hours.  The  ball  mill  was  about  half  full  of  pebbles.  A 
speedometer  indicated  the  exact  number  of  rotations  of  the  ball 
mill  and  in  this  way  each  body  received  the  same  amount  of  grinding. 

Upon  removal  from  the  ball  mill,  the  bodies  were  put 
through  a 120-mesh  screen  , only  a slight  residue  was  left  in  any 
case,  consisting  of  impurities  from  the  ballclays. 

The  dry  content  of  each  tody  was  determined  by  evaporating 
a weighed  amount  of  slip  to  dryness  and  then  reweighing* 

The  other  bodies  were  prepared  by  blending,  Nos. 2,  3 and 
4 from  Nos.l  and  5;  Nos.  11,  12  and  13  from  Nos.  1 and  10, etc. 

4.  Preparation  of  Specimens.-  Attempts  were  made  to  cast 
thin  specimens  5 cm.  by  10  cm.  and  1-2  mm.  thick,  in  a plaster 
of  paris  mould  with  the  slips  as  thus  prepared.  Trouble  was 
encountered  at  once,  due  to  the  fact  that  air  bubbles  held  in  the 
slip  , came  to  thetop  of  the  cast  specimens  and  left  a very  rough 
irregular  surface.  Attempts  to  polish  off  these  indentations 
reduced  the  specimens  to  such  a thinness , as  not  to  be  useable. 

This  trouble  was  overcane  by  pouring  each  slip  into  a 
two  liter  flask  and  exhausting  the  air  from  it  by  means  of  a vacuum 
pump.  Shading  the  slip  from  time  to  time  hastened  tne  removal 


of  the  ai r . 

It  was  found  that  slip  with  a specific  gravity  of  1.46 
to  1.50  gave  the  best  test  pieces. 

Specimens  measuring  5 crn.  by  5 cm.  and  1-2  mm.  in  thick- 
ness were  cast,  and,  upon  removal  from  the  mould  were  allowed  tc 


-9- 


dry  in  the  air  to  a bone  dry  condition.  They  were  then  preheated 
in  a small  muffle  kiln  to  100 0°C.  It  was  then  possible  to  handle 

the  specimens  without  breaking  them  and  any  irregularities  on  the 
surface  were  polished  off. 

They  were  then  imbedded  in  calcined  flint  and  burned  at 
Cone  10  in  a coal  fired  down-draft  test  kiln.  The  duration  of  the 
burn  was  36  hours. 

5.  Method  of  Procedure.-  Specimens  were  held  in  place, 
covering  the  7/3  in.  opening  in  the  photo-electric  cell  box,  by 
means  of  two  brass  damps.  7/hen  the  shutter  closed,  the  circuit 
was  completed  by  means  of  the  mercury  switch  and  a zero  reading 
was  taken  on  the  galvanometer.  The  shutter  was  then  raised, 
allowing  the  light  to  fall  directly  upon  the  specimen.  The 
light  which  was  transmitted  through  the  specimens,  fell  directly 
upon  the  photo-electric  cell  and  the  deflection  was  measured  upon 
the  galvanometer.  Ten  readings  were  taken  upon  each  specimen 
and  the  mean  of  these  taken  as  the  true  deflection.  The  circuit 
was  broken  by  "cutting  out"  the  mercury  switch,  and  the  galvanom- 
eter was  allowed  to  swing  back  to  zero  after  each  reading. 

'i-he  variations  in  the  ten  readings  were  small,  being 
no  larger  than  1 l/2  millimeter  in  any  case. 

The  thickness  of  each  specimen  was  then  measured  by 
means  of  vernier  calipers.  Measurements  were  taken  from  each  of 
the  four  corners  of  the  specimen  and  the  mean  taken  as  the  true 
thickness  of  the  specimen. 


-10- 

The  thickness  of  the  specimens  varied  from  1.2  mm.  to 
3.0  mm.  The  deflection  in  mm.  upon  the  galvanometer,  multiplied 
by  the  thickness  in  mm.  was  taxen  a.s  the  deflection  for  a thickness 
of  one  millimeter. 

it  was  found  that  130.5  volts  was  the  maximum  which  could 
be  used  with  the  photo-electric  cell.  A higher  voltage  gave 
such  a large  deflection  that  it  could  not  be  measured  upon  the 
galvanometer  scale. 


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DATA  SHEET  - TABLE  HO. 2 


-11- 


IV.  .RESULTS 


The  results  are  shown  in  Table  ho. 2.  In  specimens  of 
high  translucency  a deflection  of  about  20  mm.  was  obtained,  while 
in  specimens  of  low  translucency  the  deflection  was  as  small 
as  1.5  mm.  in  some  cases. 

It  was  found  that  with  a few  exceptions  , translucency 

was  inversely  proportional  to  the  thickness  of  the  specimen.  This 

relation  between  translucency  and  thickness  was  also  obtained  by 

9 

Ashley  and  Williams 

Graph  No.  1 with  a constant  SiO  content,  shows  that 

2 

there  is  a definite  relation  between  translucency  and  the  ratio 

feldspar  to  clay.  Translucency  increases  with  an  increase  in  the 

ratio  feldspar  to  clay,  or  we  might  say,  with  an  increase  of 

feldspar  content.  This  is  especially  true  in  bodies  of  low 

SiO  content.  A study  of  Graph  No.  1 would  seem  to  show  that  the 
2 

increase  in  translucency  with  an  increase  in  the  ratio  feldspar 

to  clay  depends  upon  the  SiO  content.  The  increase  is  very 

2 

rapid  in  low  SiO  bodies,  but  decreases  until  at  35 % SiO  only  a 

2 2 
slight  increase  is  obtained. 

Graph  No.  2 with  a constant  clay  content,  shows  that 
there  is  also  a relation  between  translucency  and  the  ratio  feld- 
spar to  oiO  yout  that  this  relation  depends  upon  the  clay  content 
2 

of  the  body.  A body  of  45^  clay  content  shows  a rapid  increase 

in  translucency  with  an  increase  in  the  ratio  feldspar  to  SiO  , 

2 

but  in  a body  of  60^  clay  content,  the  increase  is  much  smaller. 


-12- 

Graph  No.  3 with  a constant  feldspar  content  does  not  seem 
to  show  any  relationship  "between  translucency  and  the  ratio  SiO 

2 

to  clay  content. 

Figure  No.  3 is  a model  whose  height  is  regulated  by  the 
deflection  obtained  on  the  galvanometer  for  each  point  represented 
on  the  surface.  A study  of  this  model  sh ows  a gradual  increase 
in  translucency  with  an  increase  in  feldspar  content  until  a clay 
content  of  30 % is  reached.  At  this  point  there  is  a sudden  increase 
in  translucency  and  than  a gradual  increase  in  translucency  as 
the  clay  content  decreases  and  the  feldspar  increases.  This  high 
translucency  is  reached  much  sooner  upon  the  high  silica  side  of 
the  diagram.  This  sudden  increase  in  translucency  is  hard  to 
explain.  It  was  thought  that  the  presence  of  sillimanite 
crystals  might  explain  this  sudden  increase,  but  an  examination  of 
several  specimens  under  the  microscope  showed  no  trace  of  silliman- 
ite crystals.  These  could  hardly  be  expected  at  a temperature  of 
Cone  10  ( 1330°C) . 

Nr.Hecht  ^ (see  Fig. 4)  examined  bodies  containing  from  0 to 
70 % feldspar,  0 to  70%  silica  and  30  to  90%  clay.  he  found  that 
translucency  began  much  sooner  in  high  silica  bodies  than  in  low 
silica  bodies,  the  clay  remaining  constant.  This  was  verified 
as  i3  seen  in  Fig. 3 by  tne  results  obtained  in  tnis  experiment. 

Very  similar  results  were  obtained  by  various  other  investiga- 
tors . 

Ashley  ^ states  that  translucency  increases  as  the  feldspar 

is  raised  and  the  clay  substance  decreases. 

12 

Purdy  found  that  translucency  decreases  with  an 


p/o^'c  /Oatui  / 6 

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Substance  dinmnishe d-  The.  limits  shown  are  those  at  which  the  first  slight 
trans/ucency  appears  and  those  at  which  the  trans/ucency  becomes  tyoo d . 
When  the  cloy  substance  is  half  from  kao/m  and  half  from  p/astic  day 
Substance,  the  results  are  intermediate  be  twe±n  those  shown  here. 


-13- 


increase  in  clay 


Odgen 


13 


found  that  translucency  was  increased  by  keeping 


the  clay  content  low  and  increasing  the  feldspar  content. 

14 

Watts  states  that  translucency  increases  with  soda  feldspar 


c ontent . 

15 

Lin  states  that  translucency  increases  with  feldspar 
content  and  decreases  with  clay  content. 


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Figure  No.  4 


-14- 


V.  CONCLUSIONS 

1.  The  photo-electric  cell  can  he  used  to  measure 
translucency  of  thin  disks  of  ceramic  mixtures.  However,  either 
a very  sensitive  cell  or  a very  sensitive  galvanometer  w ould  he 
required  if  specimens  thicker  than  3.5  millimeters  were  used. 

Damp  weather  decreases  the  sensitiveness  of  the  photo-electric  cell, 
making  it  necessary  to  take  readings  on  clear  days. 

2.  Translucency  increases  with  an  increase  in  the  ratio 
feldspar  to  clay.  The  increase  after  a certain  point  is  rapid 

in  mixtures  of  low  silica  (15$)  content,  hut  is  slight  in  mixtures 
of  high  silica  (35$)  cron  tent.. 

3.  Translucency  increases  with  an  increase  in  the  ratio 
feldspar  to  silica,  the  increase  depending  upon  the  clay  content 
of  the  mixture.  It  is  higher  with  low  clay.  With  a clay  con- 
tent of  50$  or  above,  the  maximum  translucency  is  apparently 
reached  with  a certain  feldspar-silica,  ratio.  After  this  point 
translucency  decreases. 


-15- 


VI.  BIBLIOGRAPHY 


1 - The  Law  of  Photo-electric  Photometry.  Kunz,  J. 

"The  Astrophysical  Journal,  XLV , 3,  (1917) 

2 - Photo-electricity  . hughes 

Longmans'  jreen  and  Co.,  London,  (1913) 

3 - Photo-electricity.  Allen 

Cambridge  University  Press.  London.  (1914) 

4 - Ions,  Electrons  and  Ionizing  Radiations.  Crouther 

Edward  Arnold.  London.  (1919) 

5 - Transactions  of  the  American  Ceramic  Society 

6 - Journal  of  the  American  Ceramic  Society 

7 - Pottery  Industry 

Bureau  of  Commerce,  pg . 153 


-16- 


VII.  REFERENCES 


1 

Ashley.  Trans.  American  Ceramic  Society,  8,  149  (1906) 

Pg-2 

2 

Ogden  . Trans.  American  Ceramic  Society,  13,  400  (1911) 

Pg  .2 

3 

Williams  and  Ashley.  Trans.  American  Ceramic  society, 13,  102 

Pg . 2 (1911) 

4 

Parmelee  and  Baldwin.  Trans.  American  Ceramic  Society. 

15 , 532  (1913) . Pg  • 3 

5 

Watts.  Trans.  American  Ceramic  Society.  16,  212  (1914) 

Pg-3 

6 

Lin  . Trans.  American  Ceramic  Society,  2,  622  (1919) 

Pg-3 

7 

Bleininger.  Trans.  American  Ceramic  Society,  15,  535  (1913) 

Pg  • 3 
3 

Priest*  • Trans.  American  Ceramic  Society.  17,  150  (1915) 

Pg  * 4 
9 

Ashley  and  Williams.  Trans.  .American  Ceramic  Society.  13,  102 


10 

hecht . 

Trans . 

Pg.ll 
Am  e r i c an 

(1911) 

Ceramic  Society. 

13, 

258 

( 1911) 

11 

Ashley 

. Trans. 

Pg  • 12 
American 

Ceramic 

Society , 

13, 

253 

(1911) 

12 

Purdy. 

Trans . 

American 

Ceramic 

Soci ety , 

13, 

479 

(1911) 

13 

Ogden . 

■“■  ran  s. 

Pg  • 12 
American 

Ceramic 

Society , 

13, 

400 

( 1911) 

14 

Watts. 

T rans . 

Pg  • 13 
American 

Cerami c 

Society, 

16, 

212 

( 1914) 

15 

Lin . 

Jour.  American  Ceramic  Society,  2 

, 622 

